1. Field of the Invention
Embodiments discussed herein are directed to a radio communication apparatus and method. The embodiments particularly relate to a radio communication apparatus that conducts radio communication over MIMO downlink channels in precoding transmission under the state where each transmit antenna or each transmit antenna group has a transmit power limit.
2. Description of the Related Art
In recent years, a MIMO (Multiple-Input Multiple-Output) transmission scheme has been intensively investigated to improve frequency or spectral utilization efficiency. In this MIMO transmission scheme, for improved throughput, precoding transmissions have been intensively investigated for both a SU (Single User)-MIMO system and a MU (Multi User)-MIMO system designed to apply the MIMO transmission scheme to multiple users. In accordance with the MIMO scheme, communications are made between a communication terminal having multiple antennas and a base station having multiple antennas. In the SU-MIMO scheme, one communication terminal communicates to a base station. In the MU-MIMO scheme, multiple communication terminals communicate to a base station simultaneously.
In a unitary precoding scheme commonly used in a SU-MIMO system, a transmitter must calculate power amounts allocated to respective transmit streams under a fixed total amount of transmit power. In the case where the unitary precoding scheme is employed, a power amplifier capable of generating the total amount of transmit power must be deployed for each transmit antenna. The deployment of the power amplifiers capable of generating the total amount of transmit power at the respective transmit antennas may increase costs of implementing a high-order MIMO system such as a 8×8 MIMO system. For this reason, it is being discussed to provide transmit power limits for the respective transmit antennas and calculate power amounts allocated to different transmit streams.
On the other hand, in a MU-MIMO system, coordinate transmission between base stations in accordance with the MU-MIMO scheme is studied, and significant improvement of the throughput is reported. In the system conducting the coordinate transmission between base stations in accordance with the MU-MIMO scheme, different power amplifiers may be used for different antennas in the base stations or for the different base stations. As a result, characteristics of the respective power amplifiers must be taken into account.
In the above-stated MIMO system, a base station performs precoding on data stream to transmit to respective users based on feedback information, such as channel information, from the users. Based on the feedback information from the users, the base station performs the precoding on the data streams to transmit to the respective users. In the precoding, whenever the feedback information such as the channel information is updated, the base station needs to control transmit power allocated to the transmit streams depending on precoding weights for transmit antennas due to the transmit power limits of the respective transmit antennas in the base station or base stations.
A first article G. J. Foschini, K. Karakayali and R. A. Valenzuela, “Coordinating multiple antenna cellular networks to achieve enormous spectral efficiency”, IEEE Proceedings Communications, vol. 153, No. 4, pp. 548-555, August 2006 discloses that a Zero-forcing precoding scheme, which is a precoding scheme where transmissions are conducted to avoid interference between users, is utilized in the system for conducting the coordinate transmission between base stations in accordance with the MU-MIMO scheme in consideration of the characteristics of respective power amplifiers. This article exhibits an optimization problem on transmit power such that the respective users have even channel capacities. The first article discloses that solving the optimization problem can significantly improve the channel capacity compared with the case where the coordinate transmission is not conducted between the base stations.
A second article S. Liu, N. Hu, Z. He, K. Niu and W. Wu, “Multi-level zero-forcing method for multiuser downlink system with per-antenna power constraint”, VTC 2007-Spring, pp. 2248-2252, April 2007 discloses an optimization problem on transmit weights and transmit power for precoding in the MU-MIMO coordinated base stations transmission system such that the channel capacity of the overall system can be maximized. In this article, it is also shown that analyzing the optimization problem can improve the channel capacity.
A third article W. Yu and T. Lan, “Input optimization for multi-antenna broadcast channels with per-antenna power constraints”, IEEE GLOBECOM2004, vol. 1, pp. 420-424, December 2004 discloses an optimization problem using uplink and downlink duality and a solution thereof in the MU-MIMO coordinated base stations transmission system. This article exhibits that analyzing the optimization problem can improve convergence speed.
A fourth article Y. Ohwatari, A. Benjebbour, J. Hagiwara, and T. Ohya, “Reduced-complexity transmit power optimization techniques for multiuser MIMO with per-antenna power constraint”, Allerton2008, pp. 34-38, September 2008 discloses an optimization problem on transmit power in a MU-MIMO precoding system such that channel capacity is maximized by using one linear precoding scheme a block diagonalization Zero-forcing (BD-ZF) scheme. This article exhibits a method for sequentially updating barrier parameters for an interior point method in the case where a combination of the interior point method and a steepest descent method is utilized for optimization, an initial transmit power setup method depending on distribution of eigenvalues and an initial transmit power setup method using transmit weight information as prior information. The fourth article discloses that solving the optimization problem can improve convergence speed.